In recent years, power generation devices such as a photovoltaic power generation device, a fuel cell, a wind power generation device and a cogeneration device are used in power consumer dwellings such as houses, factories and the like. In addition, there is practically available a grid connection system in which power generation devices are connected to an electrical grid of a commercial power source and to be used as distributed power sources. In the grid connection system, if the electricity power generated by the distributed power sources cannot be entirely consumed in a consumer dwelling, the surplus electric power is allowed to be reversely fed to an electrical grid of a commercial power source. In a case where the electric power is reversely fed to the electrical grid, it is sometimes possible to receive compensation corresponding to the amount of the reversely-fed electric power from a power supplier that supplies electric power as a commercial power source.
However, if the reverse power feeding is performed in the event that the distributed power sources are connected to the electrical grid, a voltage rise, a frequency change and a phase advance in the electrical grid may possibly occur. In particular, these kinds of phenomena easily occur when the number of distributed power sources connected to an electrical grid sharing a pole transformer with the distributed power sources gets increased. Moreover, the voltage of the electrical grid is more likely to rise as the distance from the pole transformer to the distributed power sources grows longer.
In order to avoid the aforementioned problem posed when the distributed power sources are connected to the electrical grid, it is obligated to provide a function by which output control is performed as the voltage of the electrical grid rises during the reverse power feeding. There is proposed a technique in which a voltage triggering output control is set higher as the distance from a pole transformer to distributed power sources grows longer (see, e.g., Japanese Patent No. 4266003 (JP4266003B).
The aforementioned phenomena are monitored by the distributed power sources. If the phenomena occur more heavily than a specified degree, the reverse power feeding from the distributed power sources to the electrical grid is restrained to thereby reduce the influence on the electrical grid. In the technique disclosed in JP4266003B, the voltage triggering output control is set differently depending on the distance from the pole transformer to the distributed power sources so that the opportunities for the distributed power sources to perform the reverse power feeding to the electrical grid can be impartially distributed to the respective distributed power sources. In other words, the opportunities for the distributed power sources to sell a surplus power can be impartially allotted to the distributed power sources regardless of the distance from the pole transformer to the distributed power sources.
In the technique disclosed in JP4266003B, threshold values are set at the voltage triggering output control. The threshold values are adjusted depending on the impedance from the pole transformer to the distributed power sources, thereby reducing the imbalance in the opportunities for the reverse power feeding to the electrical grid. Therefore, if the number of distributed power sources connected to the secondary side of the pole transformer grows larger in a crowded area of houses, the difference between the threshold values which are set with respect to the adjoining distributed power sources becomes smaller. This requires high accuracy in setting the threshold values.
In order to set the threshold values stated above, it is necessary to calculate the impedance between the pole transformer and the distributed power sources connected to the secondary side of the pole transformer. For that reason, when installing the distributed power sources, it is necessary to acquire the information on the topology of distribution networks, the distance of wiring lines and the kind of wiring lines. It is also necessary to acquire the information on the specifications of the distributed power sources. In other words, there is a need to manage an increase amount of information. This leads to an increase in data management costs.
As set forth above, the technique disclosed in JP4266003B poses a problem in that it cannot be implemented under the condition that the number of the distributed power sources connected to the secondary side of the pole transformer is large and the distances between the adjoining distributed power sources and the electric circuit of the electrical grid are short.
If the control for restraining the reverse power feeding is performed depending on the circumstances of the electrical grid as stated above, the reverse power feeding to the electrical grid cannot be carried out even when surplus power is generated in a consumer dwelling. In other words, the consumer dwelling cannot receive compensation resulting from the reverse power feeding and, therefore, may suffer from an economic loss. As a result, the recovery period of the capital invested in installation of the distributed power sources is prolonged and the cost-effectiveness is reduced. This is one of causes weakening the motivation to adopt the distributed power source.
The reason for performing the output control of the distributed power source is that electric power is reversely fed from the distributed power source to the electrical grid. Therefore, it appears that the problem attributable to the output control of the distributed power source can be solved by reducing the reverse power feeding from the distributed power source to the electrical grid. As a technique for reducing the reverse power feeding from the distributed power source to the electrical grid, Japanese Patent Application Publication No. 2009-268247 (JP2009-268247A) discloses a method of using a battery in combination with a distributed power source. In the technique disclosed in JP2009-268247A, the moving average of the electric power amount in the past reverse power feeding is used as a target value of the electric power amount to be subjected to the reverse power feeding, thereby reducing the peak value per unit time of the electric power amount when performing the reverse power feeding.
The use of the technique disclosed in JP2009-268247A makes it possible for the battery to store the electric power not to be subjected to the reverse power feeding. This reduces the economic loss suffered by a consumer dwelling, as compared with a case where the output control of the distributed power source is performed.
However, there occurs an economic loss corresponding to the difference between the unit power price at the time of performing the reverse power feeding to the electrical grid and the power purchase price at the time of discharging and using the charged electric power. Now, it is assumed that the unit power price in case of reversely feeding electric power to the electrical grid is 48 yen/kWh and further that the unit power price in case of receiving electric power from the electrical grid is 28 yen/kWh. In that case, a profit of 48 yen per kWh is generated if the reverse power feeding is performed. Therefore, if the expense of 28 yen is subtracted from 48 yen, the actual profit becomes 20 yen. In the configuration of JP2009-268247A, the electric power of the battery storing the electric power generated in the distributed power source is discharged and used. For that reason, the expense of 28 yen per kWh becomes unnecessary. Since, however, the reverse power feeding is not performed, no profit is generated. Therefore, as compared with a case where the reverse power feeding is performed, a loss of 20 yen per kWh is incurred.
JP2009-268247A describes that the electric power amount reversely fed to the electrical grid is measured and the value obtained by equally allotting the measured electric power amount to the distributed power sources of all the consumers is regarded as an upper limit value when each of the consumers performs the reverse power feeding. However, there exists a difference in the electric power generated by the respective distributed power sources. In addition, the consumer dwellings consume different amounts of electric power. This means that there exists a difference in the surplus powers of the respective consumer dwellings. For that reason, if the measured electric power amount is equally allotted to the distributed power sources, the consumer dwelling having a larger amount of surplus power suffers a greater loss.